The present invention relates to down-the-hole tools, especially percussion drilling equipment for drilling holes in earth and rock formations.
The drilling of holes in hard subterranean formations is often performed by percussion drilling equipment, such as so-called down-the-hole hammers wherein a reciprocating hammer piston situated in the drill string immediately above the drill bit imparts percussive blows to the drill bit to promote the cutting action. The hammer position reciprocates within a cylindrical piston case which is connected at its lower end to a driver sub and at its upper end to a rear sub.
The cuttings are removed from the hole by a fluid, such as air (or liquid), which is conducted downwardly through the drill string and then ejected from a front end of the drill bit to cool the latter and entrain cuttings. The air, together with cuttings entrained therein, is conducted upwardly through an annular space formed between the outer surface of the tool and the inside wall of the hole being drilled. As a result, the outer surface of the tool, especially that of the piston case, is subjected to severe abrasion and erosion due to contact with the abrasive cuttings.
Once the piston case has been excessively worn, it must be replaced, resulting in lost drilling time and the added cost of a replacement piston case. It has been observed that the areas of the piston case outer surface which undergo the most rapid wear (and which thus determine the frequency of the replacement procedure), are the areas located at the top and bottom ends of that outer surface.
It has previously been known that areas of well drilling tools which are susceptible to wear can be protected by applying a coating of hard material to the wearsusceptible area, e.g., see U.S. Pat. No. 4,043,611. However, efforts required to apply the coating are complicated and expensive.
It has also been proposed in U.S. Pat. No. 4,706,764 to provide a cylindrical wear collar around a lower end of the driver sub to cover a portion of the latter that extends beyond a lower end of the piston casing. That proposed wear collar may include an upward extension which covers a lower portion of the piston casing. In order to secure the collar in place, the driver sub includes an upwardly facing shoulder which abuts an opposing surface of the collar to push the collar against the bottom end of the piston casing. The bottom end of the collar serves as an upward abutment surface for the drill bit. Thus, during a drilling operation the drill bit impacts against that abutment surface of the collar. Those impacts serve to place the collar under longitudinal compression, since the upper end of the collar abuts the piston casing. Therefore, the collar would have to be made of sufficient size and bulk to withstand those impacts and compression. As a result, the outer diameter of the collar would be so large (e.g., larger than the outer diameter of the piston casing) that the width of the gap formed between the collar and the side of the hole being drilled would be reduced. Such a reduction would produce an increase in the velocity of the air and cuttings flowing through that gap, thereby intensifying the rate of abrasive wear of the collar.
It would be desirable to provide an apparatus which enables the wear-susceptible areas of a piston collar to be protected in a simple manner without appreciably reducing the width of the cutting gap and without requiring the use of a large, bulky (and thus expensive) wear member.